Novel chimeric analgesic peptides

ABSTRACT

The present invention provides a novel chimeric peptide containing an opioid peptide moiety and a nociceptive peptide moiety for producing analgesia

FIELD OF THE INVENTION

The present invention relates generally to methods and compositions forthe treatment of pain. More specifically, the present invention relatesto novel chimeric peptides for the treatment of pain.

BACKGROUND OF THE INVENTION

Two million people in the United States suffer from chronic pain. Painis caused by a highly complex perception of an aversive or unpleasantsensation. The sensation of pain begins with noxious stimulation of freenerve endings, which leads to activation of different types ofnociceptive afferent fibers. These fibers include AS fibers and Cfibers. AS fibers are small diameter, thinly myelinated fibers thattransmit sharp, prickling pain. C fibers are unmyelinated and conductmore slowly and transmit dull aching pain. Repeated stimulation of painfibers can lead to hyperalgesia, or a lowering of the threshold foractivation of nociceptors.

Primary afferent fibers AS or C from the damaged periphery synapserelease a variety of chemical mediators. These mediators includeglutamate and substance P (“SP”), a nociceptive peptide. SP has longbeen recognized and identified as a neurotransmitter intimatelyassociated with the transfer of painful or nociceptive stimuli fromperipheral receptive fields into the CNS. This neuropeptide is involvedin pain signaling and the maintenance of the chronic pain state. SP isthe prototypic member of a family of related peptides named tachykinins,all of which were initially characterized by contractile activity onisolated smooth muscle preparations. SP is also found in the brain,spinal cord, spinal ganglia, and intestine of all vertebrates, includingman.

SP is present in small diameter sensory fibers that mediate nociceptiveinputs in the spinal cord, and it specifically excites nociceptiveneurons in this region. SP is released in the spinal cord in vivo, uponactivation of nociceptive primer sensory fibers. Direct application ofmicrogram doses of SP into the lumbar spinal subarachnoid produceshyperalgesia, i.e., an increased sensitivity to pain. The release of SPcan be blocked by administration of morphine and opioid peptides in vivoand in vitro. For example, intrathecal administration of morphine blocksthe hyperalgesic effects of exogenously administered SP. See, Hyden andWilcox, Eur. J. Pharmacol., 86: 95-98 (1983); and J. Pharmacol. Exp.Ther. 226: 398-404 (1983).

While opioids can be effective for the treatment of chronic pain, theyfrequently have side effects, including respiratory depression, urinaryretention, nausea and vomiting, pruritis, and sedation. Moreover,repeated daily administration of opioids eventually produces tolerance,whereby the dose of the drug must be increased in order to maintainadequate analgesia, and may also initiate physical dependence. Iftolerance develops and the level of opioids is insufficient, withdrawalsymptoms such as diarrhea, sweating, tremors, anxiety, and fever mayresult. These concerns have prompted a search for new analgesics withlimited side effects and that show decreased susceptibility totolerance.

SUMMARY OF THE INVENTION

The present invention provides a novel chimeric peptide having an opioidmoiety that binds to an opioid receptor and a nociceptive moiety thatbinds to a nociceptive receptor, such as NK₁. The opioid moiety may bedirected to any of the opioid receptor types, including the μ, δ, or κreceptor

For example, the chimeric peptide can include an Preceptor bindingopioid moiety and an NK₁-binding SP moiety. In one embodiment thischimeric peptide has the sequence: Tyr-Pro-Phe-Phe-Gly-Leu-Met-NH₂ (SEQID NO:42).

The chimeric peptides may be designed to have a plurality of SP moietiesand a plurality of opioid moieties. The plurality of opioid moieties maybe directed to the same receptor type, or, alternatively, the pluralityof opioid moieties may be directed to different opioid receptor types.

The invention provides pharmaceutical compositions including chimericpeptides and a pharmaceutically acceptable carrier useful for thetreatment of pain.

The invention also provides a method of treating pain by administeringthe chimeric peptide capable of binding to both an opioid receptor andthe NK₁ receptor admixed with a pharmaceutically acceptable carrier,such as pharmaceutical sterile saline. The peptide may be administeredintrathecally (IT), intracerebrovertricularlly (ICV) or systemically,for example, intraperitoneally (IP). Solubility of the chimeric peptidesmay be enhanced by admixtre with a solubilizing agent, for example,cyclodextran. In a alternative embodiment, a chimeric peptide isadministered in conjunction with one or more non-chimeric opioid drugs.

Among the advantages of the invention is that the chimeric peptidesproduce effective analgesia yet inhibit the development of tolerance.

The details of one or more embodiments of the invention are set forth inthe accompanying description below. Although any methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, the preferred methods andmaterials are now described. Other features, objects, and advantages ofthe invention will be apparent from the description and from the claims.In the specification and the appended claims, the singular forms includeplural referents unless the context clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which this invention belongs. Unless expressly stated otherwise,the techniques employed or contemplated herein are standardmethodologies well known to one of ordinary skill in the art. Theexamples of embodiments are for illustration purposes only. All patentsand publications cited in this specification are incorporated herein byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the chimeric peptide ESP7.

FIG. 2 is a schematic representation of the chimeric peptide ESP6.

FIG. 3 is a graph illustrating the binding affinity of ESP7 to the μreceptor.

FIG. 4 is a graph illustrating the binding affinity of ESP7 to the NK₁receptor.

FIG. 5 is a graph illustrating the analgesic effect in rats over time of1.0 μg of ESP7 administered intrathecally.

FIG. 6 is a graph illustrating the analgesic effect in rats over time of0.2 μg of ESP7 administered intrathecally.

FIG. 7 is a graph illustrating the analgesic effect in rats of 0.05 μgof ESP7 administered intrathecally.

FIG. 8 is a graph illustrating the analgesic effect in rats over time of0.2 μg of ESP7 antagonized with on days 2 and 4 with 0.2 μg ofnaltrexone.

FIG. 9 is a graph illustrating the analgesic effect in rats over time of1.0 μg of ESP7 antagonized with RP67580 on days 1-4.

FIG. 10 is a graph illustrating the analgesic effect in rats over timeof 0.1 μg of ESP7 administered intracerebroventricularly.

FIG. 11 is a graph illustrating the analgesic effect in rats over timeof 1 mg of ESP7 administered intraperitoneally.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a chimeric peptide having an opioidreceptor binding moiety and a nociceptive receptor binding moiety (e.g.,Substance P). The chimeric peptide molecules may be designed to bind toany of the opioid receptors known to be involved in pain mediation. Seereview in Lipkowski and Carr, Peptides: Synthesis, Structures, andApplications, Gutte, ed., Academic Press pp. 287-320 (1995),incorporated herein by reference. While the opioid peptides frequentlyexhibit some cross reactivity with the different receptor types, theycan be generally characterized by the degree of affinity for aparticular receptor type. These receptors include the μ receptor, the δreceptor and the κ receptor.

The separate moieties may be chemically synthesized and purified orisolated from natural sources and then chemically cross-linked to formthe chimeric peptide. Alternatively, the chimera can be chemicallysynthesized as one molecule. In another embodiment, chimeric peptidesare produced by recombinant DNA techniques and isolated from cells ortissue sources by an appropriate purification scheme using standardprotein purification techniques. The invention also relates toderivatives, fragments, homologs, analogs and variants of thesepeptides.

Chemical Synthesis

Chimeric peptides, and individual moieties or analogs and derivativesthereof, can be chemically synthesized. A variety of protein synthesismethods are common in the art, including synthesis using a peptidesynthesizer. See, e.g., Peptide Chemistry, A Practical TextbookBodasnsky, Ed. Springer-Verlag, 1988; Merrifield, Science 232: 241-247(1986); Barany, et al. Intl. J. Peptide Protein Res. 30: 705-739 (1987);Kent, Ann. Rev. Biochem. 57:957-989 (1988), and Kaiser, et al, Science243: 187-198 (1989). The peptides are purified so that they aresubstantially free of chemical precursors or other chemicals usingstandard peptide purification techniques. The language “substantiallyfree of chemical precursors or other chemicals” includes preparations ofpeptide in which the peptide is separated from chemical precursors orother chemicals that are involved in the synthesis of the peptide. Inone embodiment, the language “substantially free of chemical precursorsor other chemicals” includes preparations of peptide having less thanabout 30% (by dry weight) of chemical precursors or non-peptidechemicals, more preferably less than about 20% chemical precursors ornon-peptide chemicals, still more preferably less than about 10%chemical precursors or non-peptide chemicals, and most preferably lessthan about 5% chemical precursors or non-peptide chemicals.

Chemical synthesis of peptides facilitates the incorporation of modifiedor unnatural amino acids, including D-amino acids and other smallorganic molecules. Replacement of one or more L-amino acids in a peptidewith the corresponding D-amino acid isoforms can be used to increase theresistance of peptides to enzymatic hydrolysis, and to enhance one ormore properties of biologically active peptides, i.e., receptor binding,functional potency or duration of action. See, e.g., Doherty, et al,1993. J. Med. Chem. 36: 2585-2594; Kirby, et al., 1993. J. Med. Chem.36:3802-3808; Morita, et al., 1994. FEBS Lett. 353: 8488; Wang, et al.,1993. Int. J. Pepi. Protein Res. 42: 392-399; Fauchere and Thiunieau,1992. Adv. Drug Res. 23: 127-159.

Introduction of covalent cross-links into a peptide sequence canconformationally and topographically constrain the peptide backbone.This strategy can be used to develop peptide analogs of the chimericpeptides with increased potency, selectivity and stability. Because theconformational entropy of a cyclic peptide is lower than its linearcounterpart, adoption of a specific conformation may occur with asmaller decrease in entropy for a cyclic analog than for an acyclicanalog, thereby making the free energy for binding more favorable.Macrocyclization is often accomplished by forming an amide bond betweenthe peptide N— and C-termini, between a side chain and the N— orC-terminus [e.g., with K₃Fe(CN)₆ at pH 8.5] (Samson et al.,Endocrinology, 137: 5182-5185 (1996)), or between two amino acid sidechains. See, e.g., DeGrado, Adv Protein Chem, 39: 51-124 (1988).Disulfide bridges are also introduced into linear sequences to reducetheir flexibility. See, e.g., Rose, et al., Adv Protein Chem, 37: 1-109(1985); Mosberg et al., Biochem Biophys Res Commun, 106: 505-512 (1982).Furthermore, the replacement of cysteine residues with penicillamine(Pen, 3-mercapto-(D) valine) has been used to increase the selectivityof some opioid-receptor interactions. Lipkowski and Carr, Peptides:Synthesis, Structures, and Applications, Gutte, ed., Academic Press pp.287-320 (1995).

A number of other methods have been used successfully to introduceconformational constraints into peptide sequences in order to improvetheir potency, receptor selectivity and biological half-life. Theseinclude the use of (i) C_(α)-methylamino acids (see, e.g., Rose, et al.,Adv Protein Chem, 37:1-109 (1985); Prasad and Balaram, CRC Crit RevBiochem, 16: 307-348 (1984)); (ii) N_(α)-methylamino acids (see, e.g.,Aubry, et al., Int J Pept Protein Res, 18: 195-202 (1981); Manavalan andMomany, Biopolymers, 19: 1943-1973 (1980)); and (iii) α,β-unsaturatedamino acids (see, e.g., Bach and Gierasch, Biopolymers, 25: 5175S192(1986); Singh, et al., Biopolymers, 26: 819-829 (1987)). These and manyother amino acid analogs are commercially available, or can be easilyprepared. Additionally, replacement of the C-terminal acid with an amidecan be used to enhance the solubility and clearance of a peptide.

Recombinant Peptides

Alternatively, the peptides may be obtained by methods well-known in theart for recombinant peptide expression and purification. A DNA moleculeencoding a chimeric peptide can be generated The DNA sequence is deducedfrom the protein sequence based on known codon usage. See, e.g., Old andPrimrose, Principles of Gene Manipulation 3^(rd) ed., BlackwellScientific Publications, 1985; Wada et al., Nucleic Acids Res. 20:2111-2118(1992). Preferably, the DNA molecule includes additionalsequence, e.g., recognition sites for restriction enzymes whichfacilitate its cloning into a suitable cloning vector, such as aplasmid. The invention provides the nucleic acids comprising the codingregions, non-coding regions, or both, either alone or cloned in arecombinant vector, as well as oligonucleotides and related primer andprimer pairs corresponding thereto. Nucleic acids may be DNA, RNA, or acombination thereof. Vectors of the invention may be expression vectors.Nucleic acids encoding chimeric peptides may be obtained by any methodknown within the art (e.g., by PCR amplification using synthetic primershybridizable to the 3′- and 5′-termini of the sequence and/or by cloningfrom a cDNA or genomic library using an oligonucleotide sequencespecific for the given gene sequence, or the like). Nucleic acids canalso be generated by chemical synthesis.

The invention relates to nucleic acids hybridizable—or complementary—tothe nucleic acids encoding the chimeric peptides. In particular theinvention provides the inverse complement to nucleic acids hybridizableto the encoding nucleic acids (i.e., the inverse complement of a nucleicacid strand has the complementary sequence running in reverseorientation to the strand so that the inverse complement would hybridizewith few or no mismatches to the nucleic acid strand). Nucleic acidmolecules encoding derivatives and analogs of a chimeric peptide, orantisense nucleic acids to the same are additionally provided.

Any of the methodologies known within the relevant art regarding theinsertion of nucleic acid fragments into a vector may be used toconstruct expression vectors that contain a chimeric gene comprised ofthe appropriate transcriptional/translational control signals andpeptide-coding sequences. Promoter/enhancer sequences within expressionvectors may use plant, animal insect, or fungus regulatory sequences, asprovided in the invention.

A host cell can be any prokaryotic or eukaryotic cell. For example, thepeptide can be expressed in bacterial cells such as E coli, insectcells, fungi or mammalian cells (such as Chinese hamster ovary cells(CHO) or COS cells). Other suitable host cells are known to thoseskilled in the art In one embodiment, a nucleic acid encoding thepeptide is expressed in mammalian cells using a mammalian expressionvector. Examples of mammalian expression vectors include pCDM8 (Seed(1987) Nature 329:840) and pMT2PC (Kaulinan et al. (1987) EMBO J 6:187-195). Furthermore, transgenic animals containing nucleic acids thatencode a chimeric peptide may also be used to express peptides of theinvention.

Vector DNA can be introduced into prokaryotic or eukaryotic cells viaconventional transformation or transfection techniques. As used herein,the terms “transformation” and “transfection” are intended to refer to avariety of art-recognized techniques for introducing foreign nucleicacid (e.g., DNA) into a host cell, including calcium phosphate orcalcium chloride co-precipitation, DEAE-dextran-mediated transfection,lipofection, or electroporation. Suitable methods for transforming ortransfecting host cells can be found in Sambrook, et al. (MolecularCloning. A Laboratory Manual. 2nd ed., Cold Spring Harbor Laboratory,Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989),and other laboratory manuals.

More commonly, the host cells, can be used to produce (ie.,over-express) peptide in culture. Accordingly, the invention furtherprovides methods for producing the peptide using the host cells of theinvention. In one embodiment, the method comprises culturing the hostcell of invention (into which a recombinant expression vector encodingthe peptide has been introduced) in a suitable medium such that peptideis produced. The method further involves isolating peptide from themedium or the host cell. Ausubel et al., (Eds). In: Current Protocols inMolecular Biology. J. Wiley and Sons, New York, N.Y. 1998.

An “isolated” or “purified” recombinant peptide or biologically activeportion thereof is substantially free of cellular material or othercontaminating proteins from the cell or tissue source from which thepeptide of interest is derived. The language “substantially free ofcellular material” includes preparations in which the peptide isseparated from cellular components of the cells from which it isisolated or recombinantly produced. In one embodiment, the language“substantially free of cellular material” includes preparations ofpeptide having less than about 30% (by dry weight) of peptide other thanthe desired peptide (also referred to herein as a “contaminatingprotein”), more preferably less than about 20% of contaminating protein,still more preferably less than about 10% of contaminating protein, andmost preferably less than about 5% contaminating protein. When thepeptide or biologically active portion thereof is recombinantlyproduced, it is also preferably substantially free of culture medium,i.e., culture medium represents less than about 20%, more preferablyless than about 10%, and most preferably less than about 5% of thevolume of the peptide preparation.

Cells engineered to over-express a chimeric peptide can also beintroduced in vivo for therapeutic purposes by any method known in theart, including, but not limited to, implantation or transplantation ofcells into a host subject, wherein the cells may be “naked” orencapsulated prior to implantation. Cells may be screened prior toimplantation for various characteristics including, but not limited to,the level of peptide secreted, stability of expression, and the like.

Production of Derivatives and Analogs

The present invention also pertains to variants of the peptides thatfunction as either agonists (mimetics) or as antagonists. Variants of aparent peptides can be generated by mutagenesis, e.g., discrete pointmutation. An agonist of a parent peptide can retain substantially thesame, or a subset of, the biological activities of the naturallyoccurring form of the parent peptide. An antagonist of the peptide caninhibit one or more of the activities of the naturally occurring form ofthe parent peptide by, for example, competitively binding to thereceptor. Thus, specific biological effects can be elicited by treatmentwith a variant with a limited function. In one embodiment, treatment ofa subject with a variant having a subset of the biological activities ofthe naturally occurring form of the peptide has fewer side effects in asubject relative to treatment with the naturally occurring form of theparent peptide.

Preferably, the analog, variant, or derivative peptides are functionallyactive. As utilized herein, the term “functionally active” refers tospecies displaying one or more known functional attributes of afull-length peptide. “Variant” refers to a polynucleotide or polypeptidediffering from the polynucleotide or polypeptide of the presentinvention, but retaining essential properties thereof Generally,variants are overall closely similar, and in many regions, identical tothe polynucleotide or polypeptide of the present invention. The variantsmay contain alterations in the coding regions, non-coding regions, orboth.

Variants of the peptides that function as either agonists (mimetics) oras antagonists can be identified by screening combinatorial libraries ofmutants of the parent peptide for peptide agonist or antagonistactivity. In one embodiment, a variegated library of variants isgenerated by combinatorial mutagenesis at the nucleic acid level and isencoded by a variegated gene library. A variegated library of variantscan be produced by, for example, enzymatically ligating a mixture ofsynthetic oligonucleotides into gene sequences such that a degenerateset of potential sequences is expressible as individual peptides, oralternatively, as a set of larger fusion proteins (e.g., for phagedisplay) containing the set of sequences therein. There are a variety ofmethods which can be used to produce libraries of potential variantsfrom a degenerate oligonucleotide sequence. Chemical synthesis of adegenerate gene sequence can be performed in an automatic DNAsynthesizer, and the synthetic gene then ligated into an appropriateexpression vector. Use of a degenerate set of genes allows for theprovision, in one mixture, of all of the sequences encoding the desiredset of potential sequences. Methods for synthesizing degenerateoligonucleotides are known in the art (see, e.g., Narang (1983)Tetrahedron 39:3; Itakura et al. (1984) Annu Rev Biochem 53:323; Itakuraet al. (1984) Science 198:1056; Ike et al. (1983) Nucl Acid Res 11:477.

Derivatives and analogs of the chimeric peptides or individual moietiescan be produced by various methods known within the art For example, thepolypeptide sequences may be modified by any of numerous methods knownwithin the art. See e.g. Sambrook, et al., 1990. Molecular Cloning: ALaboratory Manual, 2nd ed, (Cold Spring Harbor Laboratory Press; ColdSpring Harbor, N.Y.). Manipulations can include by glycosylation,acetylation, phosphorylation, amidation, derivatization by knownprotecting/blocking groups, linkage to an antibody molecule or othercellular ligand, and the like. Any of the numerous chemical modificationmethodologies known within the art may be utilized including, but notlimited to, specific chemical cleavage by cyanogen bromide, trypsin,chymotrypsin, papain, V8 protease, NaBH₄, acetylation, formylation,oxidation, reduction, metabolic synthesis in the presence oftunicamycin, etc. In one embodiment, the peptide is modified by theincorporation of a heterofunctional reagent, wherein suchheterofunctional reagents may be used to connect the opioid moiety tothe nociceptive moiety.

Derivatives, fragments, and analogs provided herein are defined assequences of at least 6 (contiguous) nucleic acids or at least 4(contiguous) amino acids, a length sufficient to allow for specifichybridization in the case of nucleic acids or for specific recognitionof an epitope in the case of amino acids, respectively. Fragments are,at most, one nucleic acid-less or one amino acid-less than the wild typefill length sequence. Derivatives and analogs may be full length orother than full length, if said derivative or analog contains a modifiednucleic acid or amino acid, as described infra. Derivatives or analogsof the chimeric peptides include, but are not limited to, moleculescomprising regions that are substantially homologous in variousembodiments, of at least 30%, 40%, 50%, 60%, 70%, 80%, 90% or preferably95% amino acid identity when: (i) compared to an amino acid sequence ofidentical size; (ii) compared to an aligned sequence in that thealignment is done by a computer homology program known within the art(e.g., Wisconsin GCG software) or (iii) the encoding nucleic acid iscapable of hybridizing to a sequence encoding the aforementionedpeptides under stringent (preferred), moderately stringent, ornon-stringent conditions. See, e.g., Ausubel, et al., Current Protocolsin Molecular Biology, John Wiley and Sons, New York, N.Y., 1993.

Derivatives of the chimeric peptides may be produced by alteration oftheir sequences by substitutions, additions or deletions that result infunctionally-equivalent molecules. Thus, the invention includes DNAsequences that encode substantially the same amino acid sequence. Inanother embodiment, one or more amino acid residues within the sequenceof interest may be substituted by another amino acid of a similarpolarity and net charge, thus resulting in a silent alteration.Substitutes for an amino acid within the sequence may be selected fromother members of the class to which the amino acid belongs. For example,nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine,valine, proline, phenylalanine, tryptophan and methionine. Polar neutralamino acids include glycine, serine, threonine, cysteine, tyrosine,asparagine, and glutamine. Positively charged (basic) amino acidsinclude arginine, lysine and histidine. Negatively charged (acidic)amino acids include aspartic acid and glutamic acid.

In particular embodiments, the chimeric peptides, and fragments,derivatives, homologs or analogs thereof, are related to animals (e.g.,mouse, rat, pig, cow, dog, monkey, frog), or human opioids. Homologs(i.e., nucleic acids encoding peptides derived from species other thanhuman) or other related sequences (e.g., paralogs) can also be obtainedby low, moderate or high stringency hybridization with all or a portionof the particular human sequence as a probe using methods well known inthe art for nucleic acid hybridization and cloning. See, e.g., Ausubelet al., (eds.), 1993, Current Protocols in Molecular Biology, John Wileyand Sons, NY; and Kriegler, 1990, Gene Transfer and Expression, ALaboratory Manual, Stockton Press, NY.

In one embodiment, a nucleic acid sequence that is hybridizable to anucleic acid sequence (or a complement of the foregoing) encoding thechimeric peptides, or a derivative of the same, under conditions of highstringency is provided: Step 1: Filters containing DNA are pretreatedfor 8 hours to overnight at 65° C. in buffer composed of 6×SSC, 50 mMTris-HCl (pH 7.5), 1 mM EDTA, 0.02% PVP, 0.02% Ficoll, 0.02% BSA, and500 μg/ml denatured salmon sperm DNA. Step 2: Filters are hybridized for48 hours at 65° C. in the above prehybridization mixture to which isadded 100 mg/ml denatured salmon sperm DNA and 5-20×10⁶ cpm of³²P-labeled probe. Step 3: Filters are washed for 1 hour at 37° C. in asolution containing 2×SSC, 0.01% PVP, 0.01% Ficoll, and 0.01% BSA. Thisis followed by a wash in 0.1×SSC at 50° C. for 45 minutes. Step 4:Filters are autoradiographed. Other conditions of high stringency thatmay be used are well known in the art.

In a second embodiment, a nucleic acid sequence that is hybridizable toa nucleic acid sequence (or a complement thereof) encoding the chimericpeptides, or derivatives, under conditions of moderate stringency isprovided: Step 1: Filters containing DNA are pretreated for 6 hours at55° C. in a solution containing 6×SSC, 5× Denhardt's solution, 0.5% SDSand 100 mg/ml denatured salmon sperm DNA. Step 2: Filters are hybridizedfor 18-20 hours at 55° C. in the same solution with 5-20×106 cpm³²P-labeled probe added. Step 3: Filters are washed at 37° C. for 1 hourin a solution containing 2×SSC, 0.1% SDS, then washed twice for 30minutes at 60° C. in a solution containing 1×SSC and 0.1% SDS. Step 4:Filters are blotted dry and exposed for autoradiography. Otherconditions of moderate stringency that may be used are well-known in theart.

In a third embodiment, a nucleic acid that is hybridizable to a nucleicacid sequence disclosed in this invention or to a nucleic acid sequenceencoding a the aforementioned peptides, or fragments, analogs orderivatives under conditions of low stringency: Step 1: Filterscontaining DNA are pretreated for 6 hours at 40° C. in a solutioncontaining 35% formamide, 5×SSC, 50 mM Tris-HCl (pH 7.5), 5 mM EDTA,0.1% PVP, 0.1% Ficoll, 1% BSA, and 500 μg/ml denatured salmon sperm DNAStep 2: Filters are hybridized for 18-20 hours at 40° C. in the samesolution with the addition of 0.02% PVP, 0.02% Ficoll, 0.2% BSA, 100μg/ml salmon sperm DNA, 10% (wt/vol) dextian sulfate, and 5-20×106 cpm³²P-labeled probe. Step 3: Filters are washed for 1.5 hours at 55° C. ina solution containing 2×SSC, 25 mM Tris-HCl (pH 7.4), 5 mM EDTA, and0.1% SDS. The wash solution is replaced with fresh solution andincubated an additional 1.5 hours at 60° C. Step 4: Filters are blotteddry and exposed for autoradiography. If necessary, filters are washedfor a third time at 65-68° C. and re-exposed to film. Other conditionsof low stringency that may be used are well known in the art (e.g., asemployed for cross-species hybridizations). See also Shilo and Weinberg,Proc Natl Acad Sci USA 78: 6789-6792 (1981).

Design of Chimeric Peptides

Peptides with Affinity for the μ Receptor

The exogenous opioid peptide agonists for the 11 receptor type includethose listed in Table 1: α-endorphin, endomorphin-1, endomorphin-2,dermorphin, β-casomorphin (bovine or human), Morphiceptin,Leu-enkephalin, Met-enkephalin, DALDA, and PL107. Modifications of thepeptides have resulted in very selective μ receptor ligands. Thesemodifications can include amidation of the carboxyl terminus (—NH₂), theuse of (D) amino acids in the peptide (e.g. DALDA), incorporation ofsmall non-peptidyl moieties, as well as the modification of the aminoacids themselves (e.g. alkylation or esterification of side chainR-groups). As in, for example, the compound DAMGO:Tyr-(D)Ala-Gly-Phe-NHCH₂CH₂OH. TABLE 1 SEQ ID μ receptor NO: agonistSequence 1 α-endorphin Tyr-Gly-Gly-Phe-Met-Thr-Ser-Glu-Ser-Gln-Thr-Pro-Leu-Val-Thr-NH₂ 2 endomorphin-1 Tyr-Pro-Trp-Phe-NH₂ 3endomorphin-2 Tyr-Pro-Phe-Phe-NH₂ 4 dermorphinTyr-(D)Ala-Phe-Gly-Tyr-Pro-Ser- NH₂ 5 β-casomorphinTyr-Pro-Phe-Pro-Gly-Pro-Ile (bovine) 6 β-casomorphinTyr-Pro-Phe-Val-Glu-Pro-Ile (human) 7 Morphiceptin Tyr-Pro-Phe-Pro-NH₂ 8Leu-enkephalin Tyr-Gly-Gly-Phe-Leu 9 Met-enkephalin Tyr-Gly-Gly-Phe-Met10 DALDA Tyr-(D)Arg-Phe-Lys-NH₂ 11 PL017 Tyr-Pro-(N-Me)Phe-(D)Pro-NH₂Peptides With affinity for the δ Receptor

Other suitable opioid peptide moieties include the δ receptor agonistslisted in Table 2. Those with the highest receptor selectivity generallyare enkephalin-derived peptides. For example, DADLE has a three to tenfold higher selectivity for the δ receptor than the μ receptor.Modifications of the parent enkephalin sequence results in two groups ofpeptide analogs. The first group is a series of linear analogs, forexample, DSLET. The second group, all rigid cyclic analogs, includesDPDPE (where Pen is penicillamine, or 3-mercapto-(D)Valine). In bindingassays, these analogs show an 100-fold affinity for the δ receptor overthe μ-receptor and a 1000-fold increase over the κ-receptor. Additionalpseudopeptide analogs, either linear or cyclic, also display highselectivity to the δ receptor, for example Tyr-Tic-Phe-Phe, where Tic isL-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid Schiller, et al., J.Med. Chem. 36: 3182-3187 (1993). TABLE 2 SEQ ID δ receptor NO: agonistsSequence 12 DADLE Tyr-(D)Ala-Gly-Phe-(D)Leu 13 DSLETTyr-(D)Ser-Gly-Phe-Leu-Thr 14 DPDPE         _(———————————————————)        |               | Tyr-(D)Pen-Gly-Phe-(D)Pen 15 deltorphin ITyr-(D)Ala-Phe-Asp-Val-Val-Gly- NH₂ 16 deltorphin IITyr-(D)Ala-Phe-Glu-Val-Val-Gly- NH₂ 17 dermenkephalinTyr-(D)Met-Phe-His-Leu-Met-Asp- NH₂Peptides With Affinity for the κ Receptor

Opioid moieties also include Dynorphin (“DYN”) related peptides, whichare endogenous peptide agonists for the κ receptor. Some representativepeptides are shown in Table 3. The propeptide, pro-dynorphin, isprocessed into peptides of different lengths and with different receptorselectivities. Several of these peptides, including Dynorphin A,DYN(1-8), and DYN(1-13) are found in the CNS of vertebrates inphysiologically significant concentrations. Several dynorphin analogshave been generated by substitution of D-amino acids at position 8 (Ile)or 10 (Pro). Additionally cyclic dynorphin analogs with high K receptorselectivity have been generated: e.g.,Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Cys-Arg-Pro-Lys-Leu-Cys-NH, (SEQ ID NO: 44),where the two Cysteines are engaged in a disulfide bond, to create a sixamino acid ring. TABLE 3 SEQ ID κ receptor NO: agonists Sequence 18Dynorphin A Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln 19 DYN (1-8)Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile 20 DYN (1-13)Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg- Pro-Lys-Leu-LysPeptides With Affinity for NK₁ Receptor: Substance P Peptides

The SP moiety of the chimeric peptide is designed to bind to the NK₁receptor. SP is an 11 amino acid peptide, which has a number ofdifferent natural and synthetic analogs. A representative group is shownin Table 4, below. A number of SP amino-terminal fragments and modifiedpeptides have a high degree of specificity for the NK₁ receptor relativeto NK₂ and NK₃ receptors. This specificity can be increased byesterification of the carboxy terminal amide. Other modificationsinclude the generation of cyclic molecules (e.g. via Cys-Cys disulfidebridges), the incorporation of non-peptidyl moieties (e.g. spirolactonesas discussed by Ward in J. Med. Chem. 33: 1848-1851 (1990)).Additionally, SP and SP analogs can be made more stable by using D-aminoacids. A representative listing of SP and its related family ofcompounds is provided in Table 4 below. TABLE 4 SEQ ID NO: CompoundSequence 21 SP Arg-Pro-Lys-Pro-Gln-Gln-Phe- Phe-Gly-Leu-Met-NH₂ 22SP-Glycine Arg-Pro-Lys-Pro-Gln-Gln-Phe- Phe-Gly-Leu-Met-Gly-NH₂ 23SP-Glycine-Lysine Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-Gly-Lys-NH₂ 24 SP-Glycine-Lysine-Arg-Pro-Lys-Pro-Gln-Gln-Phe- Arginine Phe-Gly-Leu-Met-Gly-Lys-Arg- NH₂25 SP-Glycine Methyl Arg-Pro-Lys-Pro-Gln-Gln-Phe- EsterPhe-Gly-Leu-Met-Gly-O^(me) 26 SP-Glycine-Lycine-Arg-Pro-Lys-Pro-Gln-Gln-Phe- Methyl Ester Phe-Gly-Leu-Met-Gly-Lys-O^(me)27 SP-Glycine-Lysine- Arg-Pro-Lys-Pro-Gln-Gln-Phe- Arginine MethylPhe-Gly-Leu-Met-Gly-Lys-Arg- Ester O^(me) 28 SP-Glycine-ElthylArg-Pro-Lys-Pro-Gln-Gln-Phe- Ester Phe-Gly-Leu-Met-Gly-O^(eth) 29SP-Glycine-Lysine Arg-Pro-Lys-Pro-Gln-Gln-Phe- Ethyl EsterPhe-Gly-Leu-Met-Gly-Lys-O^(eth) 30 SP-Glycine-Lysine-Arg-Pro-Lys-Pro-Gln-Gln-Phe- Arginine Ethyl Phe-Gly-Leu-Met-Gly-Lys-Arg-Ester O^(eth) 31 SP/1-4# Arg-Pro-Lys-Pro-NH₂ 32 SP/1-7#Arg-Pro-Lys-Pro-Gln-Gln-Phe- NH₂ 33 SP/1-9# Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-NH₂ 34 [D-Pro2, D-Phe7, Arg-(D)Pro-Lys-Pro-Gln-Gln- D-Trp9]-SP(D)Phe-Phe-(D)Trp-Leu-Met- NH₂ 35 [D-Pro2, D-Phe7,Arg-(D)Pro-Lys-Pro-Gln-Gln- D-Trp9]-SP- (D)Phe-Phe-(D)Trp-Leu-Met-(Glycine) Gly-NH₂ 36 [D-Pro2, D-Trp7, Arg-(D)Pro-Lys-Pro-Gln-Gln-D-Trp9]-SP (D)Trp-Phe-(D)Trp-Leu-Met- NH₂ 37 [D-Pro2, D-Trp7,Arg-(D)Pro-Lys-Pro-Gln-Gln- D-Trp9]-SP- (D)Trp-Phe-(D)Trp-Leu-Met-Glycine Gly-NH₂ 38 [Cys3, Cys6, Tyr8, Arg-Pro-Cys-Pro-Gln-Cys-Phe-Pro10]-SP Tyr-Gly-Pro-Met-NH₂ 39 [Glu6]-SP/6-11Glu-Phe-Phe-Gly-Leu-Met-NH₂ 40 Septide Glu-Phe-Phe-Pro-Leu-Met-NH₂ 41Sanktide HOOC-CH₂-CH₂-CO-Asp-Phe-(N- Me)Phe-Gly-Leu-Met-NH₂

If the target of the chimeric peptide is the μ receptor, the opioidagonist moiety is chosen from those shown to be selective for thatreceptor, e.g. those in Table 1. If the opioid target receptor is the δreceptor, the opioid agonist moiety is selected from the groupconsisting of DADLE, DSLET, DPDPE, deltorphin I, deltorphin II anddermenkephalin. If the target opioid receptor of the chimeric peptide isthe K-receptor, the opioid agonist moiety is selected from the groupconsisting of the dynorphin peptides. The chimeric peptide may besynthesized to have a plurality of opioid moieties. These opioidmoieties may be directed to any combination of the opioid receptors ormay be directed to the same receptor type. Furthermore, a chimera may besynthesized to contain a plurality of SP moieties per each opioidmoiety. In one embodiment, the novel chimeric peptide is ESP7, SEQ IDNO:42 (FIG. 1). Because it includes endomorphin-2 at the N-terminus andSP (7-11) at the C-terminus, ESP7 is designed to bind to the μ receptorand the NK₁ receptor. One ESP7 derivative is ESP6, or Pro 5 ESP7:Tyr-Pro-Phe-Phe-Pro-Leu-Met-NH₂ (FIG. 2, SEQ ID NO:43).

Pharmaceutical Compositions

The chimeric peptides of the invention, and derivatives can beincorporated into pharmaceutical compositions suitable foradministration. Such compositions typically comprise the peptide and apharmaceutically acceptable carrier. As used herein, “pharmaceuticallyacceptable carrier” is intended to include any and all solvents,dispersion media, coatings, antibacterial and antifungal agents,isotonic and absorption delaying agents, and the like, compatible withpharmaceutical administration. The use of such media and agents forpharmaceutically active substances is well known in the art Exceptinsofar as any conventional media or agent is incompatible with theactive compound, use thereof in the compositions is contemplated.Supplementary active compounds can also be incorporated into thecompositions. Modifications can be made to the peptide of the presentinvention to affect solubility or clearance of the peptide. Thesemolecules may also be synthesized with D-amino acids to increaseresistance to enzymatic degradation. If necessary, the chimeric peptidescan be co-administered with a solubilizing agent, such as cyclodextran.

A pharmaceutical composition of the invention is formulated to becompatible with its intended route of administration. Examples of routesof administration include parenteral, e.g., intravenous, intradermal,subcutaneous, oral (e.g. inhalation), transdermal (topical),transmucosal, and rectal administration. Solutions or suspensions usedfor parenteral, rintradernal, or subcutaneous application can includethe following components: a sterile diluent such as water for injection,saline solution, fixed oils, polyethylene glycols, glycerine, propyleneglycol or other synthetic solvents; antibacterial agents such as benzylalcohol or methyl parabens; antioxidants such as ascorbic acid or sodiumbisulfite; chelating agents such as ethylenediaminetetraacetic acid;buffers such as acetates, citrates or phosphates, and agents for theadjustment of tonicity such as sodium chloride or dextrose. The pH canbe adjusted with acids or bases, such as hydrochloric acid or sodiumhydroxide. The parenteral preparation can be enclosed in ampoules,disposable syringes or multiple dose vials made of glass or plastic.

Pharmaceutical compositions suitable for injectable use include sterileaqueous solutions (where water soluble) or dispersions and sterilepowders for the extemporaneous preparation of sterile injectablesolutions or dispersion. For intravenous administration, suitablecarriers include physiological saline, bacteriostatic water, CremophorEL™ (BASF, Parsippany, N.J.) or phosphate buffered saline (PBS). In allcases, the composition must be sterile and should be fluid to the extentthat easy syringability exists. It must be stable under the conditionsof manufacture and storage and must be preserved against thecontaminating action of microorganisms such as bacteria and fungi. Thecarrier can be a solvent or dispersion medium containing, for example,water, ethanol, polyol (for example, glycerol, propylene glycol, andliquid polyethylene glycol, and the like), and suitable mixtures thereofThe proper fluidity can be maintained, for example, by the use of acoating such as lecithin, by the maintenance of the required particlesize in the case of dispersion and by the use of surfactants. Preventionof the action of microorganisms can be achieved by various antibacterialand antifungal agents, for example, parabens, chlorobutanol, phenol,ascorbic acid, thimerosal, and the like. In many cases, it will bepreferable to include isotonic agents, for example, sugars, polyalcoholssuch as manitol, sorbitol, sodium chloride in the composition. Prolongedabsorption of the injectable compositions can be brought about byincluding in the composition an agent which delays absorption, forexample, aluminum monostearate and gelatin.

Sterile injectable solutions can be prepared by incorporating the activecompound (e.g., chimeric peptide) in the required amount in anappropriate solvent with one or a combination of ingredients enumeratedabove, as required, followed by filtered sterilization. Generally,dispersions are prepared by incorporating the active compound into asterile vehicle that contains a basic dispersion medium and the requiredother ingredients from those enumerated above. In the case of sterilepowders for the preparation of sterile injectable solutions, methods ofpreparation are vacuum drying and freeze-drying that yields a powder ofthe active ingredient plus any additional desired ingredient from apreviously sterile-filtered solution thereof

Oral compositions generally include an inert diluent or an ediblecarrier. They can be enclosed in gelatin capsules or compressed intotablets. For the purpose of oral therapeutic administration, the activecompound can be incorporated with excipients and used in the form oftablets, troches, or capsules. Oral compositions can also be preparedusing a fluid carrier for use as a mouthwash, wherein the compound inthe fluid carrier is applied orally and swished and expectorated orswallowed. Pharmaceutically compatible binding agents, and/or adjuvantmaterials can be included as part of the composition. The tablets,pills, capsules, troches and the like can contain any of the followingingredients, or compounds of a similar nature: a binder such asmicrocrystalline cellulose, gum tragacanth or gelatin; an excipient suchas starch or lactose, a disintegrating agent such as alginic acid,Primogel, or corn starch; a lubricant such as magnesium stearate orSterotes; a glidant such as colloidal silicon dioxide; a sweeteningagent such as sucrose or saccharin; or a flavoring agent such aspeppermint, methyl salicylate, or orange flavoring.

For administration by inhalation, the compounds are delivered in theform of an aerosol spray from pressured container or dispenser whichcontains a suitable propellant, e.g., a gas such as carbon dioxide, or anebulizer.

Systemic administration can also be by transmucosal or transdermalmeans. For transmucosal or transdermal administration, penetrantsappropriate to the barrier to be permeated are used in the formulation.Such penetrants are generally known in the art, and include, forexample, for transmucosal administration, detergents, bile salts, andfusidic acid derivatives. Transmucosal administration can beaccomplished through the use of nasal sprays or suppositories. Fortransdermal administration, the active compounds are formulated intoointments, salves, gels, or creams as generally known in the art.

The compounds can also be prepared in the form of suppositories (e.g.,with conventional suppository bases such as cocoa butter and otherglycerides) or retention enemas for rectal delivery.

In one embodiment, the active compounds are prepared with carriers thatwill protect the compound against rapid elimination from the body, suchas a controlled release formulation, including implants andmicroencapsulated delivery systems. Biodegradable, biocompatiblepolymers can be used, such as ethylene vinyl acetate, polyanhydrides,polyglycolic acid, collagen, polyorthoesters, and polylactic acid.Methods for preparation of such formulations will be apparent to thoseskilled in the art The materials can also be obtained commercially fromAlza Corporation and Nova Pharmaceuticals, Inc. Liposomal suspensions(including liposomes targeted to infected cells with monoclonalantibodies to viral antigens) can also be used as pharmaceuticallyacceptable carriers. These can be prepared according to methods known tothose skilled in the art, for example, as described in U.S. Pat. No.4,522,811.

It is especially advantageous to formulate oral or parenteralcompositions in dosage unit form for ease of administration anduniformity of dosage. Dosage unit form as used herein refers tophysically discrete units suited as unitary dosages for the subject tobe treated; each unit containing a predetermined quantity of activecompound calculated to produce the desired therapeutic effect inassociation with the required pharmaceutical carrier. The specificationfor the dosage unit forms of the invention are dictated by and directlydependent on the unique characteristics of the active compound and theparticular therapeutic effect to be achieved, and the limitationsinherent in the art of compounding such an active compound for thetreatment of individuals.

Nucleic acid molecules encoding the chimeric peptides of the inventioncan be inserted into vectors and used as gene therapy vectors. Genetherapy vectors can be delivered to a subject by, for example,intravenous injection, local administration (see U.S. Pat. No.5,328,470) or by stereotactic injection (see e.g., Chen et al. (1994)PNAS 91:3054-3057). The pharmaceutical preparation of the gene therapyvector can include the gene therapy vector in an acceptable diluent, orcan comprise a slow release matrix in which the gene delivery vehicle isimbedded. Alternatively, where the complete gene delivery vector can beproduced intact from recombinant cells, e.g., retroviral vectors, thepharmaceutical preparation can include one or more cells that producethe gene delivery system.

The pharmaceutical compositions can be included in a container, pack, ordispenser together with instructions for administration.

Treatment of Pain

The invention further provides methods of treating a mammal for pain byadministering a pharmaceutical composition (as described above) in orderto produce analgesia in the patient One method to assess the analgesicproperties of the chimeric peptides is the tail flick test, which isadministered to rats following intrathecal, intracerebroventricular, andintraperitoneal administration. The effects of opioid antagonists (e.g.,naltrexone) and NK₁ antagonists (e.g., RP67580) on the activity of thepeptides can be assessed according to methods common in the art.

In order that this invention may be better understood, the followingexamples are set forth These examples are for the purposes ofillustration only and are not to be construed as limiting the scope ofthis invention in any manner.

EXAMPLE 1 In Vitro Binding of ESP7 to Opioid and SP Receptors in RatBrain Preparations

In order to assess the binding affinity of ESP7 to opioid and SPreceptor, binding assays to opioid and SP receptors were performed withcrude rat brain plasma membranes prepared using a modified procedure ofZadina Zadina et al., Life Sci, 55: 461-466 (1994). These assays showedthat ESP7 has a strong affinity for both the μ receptor and the NK₁receptor in rat brain.

For binding assays to opioid receptors, frozen rat brains (−80° C.) werehomogenized in 40 volumes of standard Tris buffer (50 mM Tris HCl (pH7.4), 0.2 mg/ml BSA, 2.5 mM EDTA, 40 μg/ml bacitracin, 30 μg/ml bestatinand 5 mM MgCl₁) and centrifuged at 15,000×g for 20 minutes. 100 mM NaClwas added to the buffer, in order to remove endogenous ligands, and thecentrifugation was repeated After a wash with standard buffer, themembrane preparation was finally resuspended in 10 volumes of incubationbuffer (standard buffer with 4 μg/ml leupeptin and 2 μg/ml chymostatin).The same procedure was followed for the SP receptor except the wash withNaCl was eliminated and 5 mM MgCl₂ was replaced with 3 mM MnCl₂. Thebrain homogenates were used on the day of preparation.

Binding assays for the μ receptor were performed at 4° C. for 90minutes, as described in Zadina et al., Life Sci, 55: 461-466 (1994). Afinal volume of 0.35 mL was used which contained incubation buffer(described above), brain homogenate, and 1.85 nM [³H]DAMGO with orwithout competing peptide (DAMGO or ESP7). Nonspecific binding wasdetermined with 10 μM DAMGO. After incubation the samples were filteredon a Brandell-Harvester using an appropriate GF/B filter soaked in 50 mMTris HCl (pH 7.4) and 0.5% PEI. Scintillation fluid was added to thefilters in order to solubilize the membranes, and a beta-counter wasused to quantify radioactivity.

A procedure similar to the opioid binding assay was followed for the SPreceptor except the SP assays were performed at room temperature for 75minutes using 23 fmol of [¹²⁵I]BH-SP. 10 μM SP defined the non-specificbinding. After filtration, the radioactivity was determined on agamma-counter.

As seen in the FIG. 3, DAMGO had a K_(d) of approximately 3 nM (FIG. 8).ESP7 had a K_(d) of approximately 300 nM, illustrating that ESP7possesses significant affinity for the μ receptor. As seen in FIG. 4, SPhad a K_(d) of approximately 0.03 nM, while ESP7 had a K_(d) ofapproximately 200 nM. Thus, ESP7 has significant affinity for the NK₁receptor, and, as expected, ESP7 bound specifically to and hadsignificant binding affinity for both the μ receptor and the NK₁receptor.

EXAMPLE 2 Characterization of the Analgesic Properties of ESP7

ESP7 was tested clinically in rats to determine analgesic effect andtolerance. The classical tail flick test was used to measure painresponse and thermal pain was mimicked using a heat source. This systemwas controlled using standard opioids. The drug was administered withcyclodextran to increase solubility of the peptide in an aqueoussolution.

2.1 Intrathecal Administration of ESP7 in Rats and the Effects ofNaltrexone and RP67580 Blockades

Intrathecal administration of ESP7 produced long-lasting analgesiawithout any significant development of tolerance. The opioid antagonistnaltrexone blocked this analgesia, indicating that the analgesia wasopioid in nature. Additionally, when the SP portion was antagonized withRP67580, an NK₁ antagonist, tolerance to the drug developed within threedays. These results indicate that the SP moiety of ESP7 does notcontribute to the analgesia, but rather plays an integral role inpreventing the development of tolerance.

Adult male Sprague Dawley rats (200-250 g) were implanted with chronicindwelling intrathecal catheters using a modified protocol of Yaksh andRudy, Physiol. Behav., 17: 1031-1036 (1976). Catheters were made ofsilastic tubing, had an inside diameter of 0.012″ and an outsidediameter of 0.025″, and measured a total of 11.5 cm with 7.5 cm insertedinto the intrathecal space to level T13-LI. The rats were anesthetizedthroughout the surgery with 5.0% isoflurane. The catheter was insertedthrough the alanto-occipital membrane and into the intrathecal spaceusing a guide wire. Sutures were used to secure the placement of thecatheter. The rats were allowed to recover from surgery for 3-4 days andany rats with neurological impairment were not used for analgesicmeasurements. Rats were housed separately in a 12 hr light-dark cyclewith free access to food and water. During their recovery from surgery,rats were habituated to the laboratory environment and analgesic testingapparatus.

For measurement of the thermal anti-nociceptive properties of thepeptides of interest, the tail flick test was employed. Rats were firsthabituated to the tail flick chamber. During testing, the rats wereplaced in the chamber and a light source, which generated heat, wasdirected at their tail. The latency to remove the tail was recorded. Thebaseline latency was approximately 3.5 sec and the cutoff latency was 10sec to avoid tissue damage. Three measurements were made at each pre-and post-treatment time point and the results were averaged. Responseswere expressed as % maximum possible effect:${\%\quad{MPE}} = {\frac{\text{post-treatment~~latency} - \text{baseline~~latency}}{\text{cutoff~~time} - \text{baseline}} \times 100}$After testing, the rats were sacrificed and the correct placement of thecatheter was verified by dissection of the spinal cord.

Rats were given doses of 1.0 μg (FIG. 5), 0.2 μg (FIG. 6), and 0.05 μg(FIG. 7) of ESP7. The desired concentration of the compound (in 10 μl)was injected into the catheter followed by 10 μl saline flush to fillthe dead volume. ESP7 was combined with two molecules of cyclodextrin(ESP7+2CD) to increase solubility. Ultimately, cyclodextrin can formreversible complexes with lipophilic compounds such as ESP7 to increasetheir solubility, decrease their clearance from the spinal cord andenhance their duration of action.

As shown in FIG. 5, the 1.0 μg dose produced a low level of prolongedanalgesia for five days. More interestingly, no tolerance developed tothe effects of ESP7+2CD (p>0.05). As shown in FIG. 6, the analgesiaproduced by 0.2 μg remained at the same level for five days (p>0.05). Asshown in FIG. 7, however, some tolerance did appear to develop at the0.05 μg dose on day 5 (p=0.014). As a control 1.0 μg of 2-cyclodextrinwas administered intrathecally with no significant effect (p>0.05) (datanot shown).

To examine whether the analgesia produced was opioid in nature, theopioid receptor was antagonized with naltrexone. On the days indicatedbelow, naltrexone, was administered 10 min prior to ESP7+2CD. As shownin FIG. 8, 0.2 μg ESP7+2CD produced analgesia on Days 1, 3, and 5, butnot on Days 2 and 4 when naltrexone was administered (p=0.0042). Similarresults were seen with the 1.0 μg of ESP7+2CD, where naltrexone againsignificantly blocked the analgesia (p=0.0009) (data not shown).Naltrexone actually produced some hyperalgesia when given with bothdoses of ESP7+2CD, unmasking the nociceptive activity of SP. Inaddition, the naltrexone blockade was reversible once the drug had beenremoved. A control experiment illustrated that naltrexone alone producedno change in analgesia (p>0.05)(data not shown).

To examine whether the reduced tolerance exhibited in rats treated withESP7 was SP mediated, the NK₁ receptor was antagonized with RP67580, aspecific NK₁ antagonist with high affinity for the rat NK₁ receptor.RP67580 (250 pmol) was administered IT prior to 1.0 μg ESP7+2CD. As seenin FIG. 9, ESP7+2CD produced significant analgesia on Day 1, buttolerance developed to this analgesia within three days (p<0.0001).Slight hyperalgesia was present on Day 4. On Day 5, the NK₁ antagonistwas removed and a partial rescue of the analgesia occurred. The level ofanalgesia on Day 5 reached a level similar to Day 2 (p>0.05). RP67580alone had no effect on the level of analgesia (p>0.05). Therefore, ESP7administered intrathecally was able to induce analgesia while minimizingthe development of tolerance.

2.2 Intracerebroventricular Administration of ESP7 in Rats

Adult male Sprague Dawley rats weighing 200-250 g were use Beforesurgery, the rats were anesthesized with 0.2-0.3 mL of xylazine (10%)and ketamine (90%). Rats were positioned in a stereotaxic apparatus andthe bregma was located. To reach the lateral ventricle, a hole wasdrilled 0.8 mm caudal and 1.4-1.5 mm left or right of the bregma. Thecatheter was inserted 4.5 mm deep into the brain and 4.0 cm ofpolyethylene tubing was connected to the end to the catheter. Screws anddental cement were used to secure the catheter in place. After suturingthe skin, the rats were allowed to recover from surgery for 4-5 days.Each rat was housed separately in a 12 hr light-ark cycle with freeaccess to food and water. Rats with any neurological problems were notused in the analgesic testing.

The tail flick assay was used to measure analgesia as described above inExample 2.1. Briefly, the rats were first habituated to the tail flickchamber. During testing, the rats were placed in the chamber and a lightsource, which generated heat, was directed at their tail. The latency toremove the tail was recorded The baseline latency was approximately 3.5sec and the cutoff latency was 10 sec to avoid tissue damage. Threemeasurements were made at each pre- and post-treatment time point andthe results were averaged. Responses were expressed as % maximumpossible effect (MPE):${\%\quad{MPE}} = {\frac{\text{post-treatment~~latency} - \text{baseline~~latency}}{\text{cutoff~~time} - \text{baseline}} \times 100}$After resting the rats were sacrificed and the correct placement of thecatheter was verified.

As shown in FIG. 10, 0.1 μg ESP7+2CD produced a low level of analgesiathat dissipated after one hour. ESP7 (1.0 μg) also produced analgesia(data not shown).

2.3 Intraperitoneal Administration of ESP7 in Rats

ESP7 was administered intraperitoneally in order to assess theeffectiveness of ESP7 systemically. The tail flick assay was used tomeasure analgesia as described above in Example 2.1. Briefly, the ratswere first habituated to the tail flick chamber. During testing, therats were placed in the chamber and a light source, which generatedheat, was directed at their tail. The latency to remove the tail wasrecorded. The baseline latency was approximately 3.5 sec and the cutofflatency was 10 sec to avoid tissue damage. Three measurements were madeat each pre- and post-treatment time point and the results wereaveraged. Responses were expressed as % maximum possible effect (MPE):${\%\quad{MPE}} = {\frac{\text{post-treatment~~latency} - \text{baseline~~latency}}{\text{cutoff~~time} - \text{baseline}} \times 100}$

As seen in FIG. 11, 1 mg of ESP7 produced analgesia as seen withintrathecal administration. In addition, 3 mg of ESP7+2CD also producedanalgesia similar to that seen with intrathecal administration (data notshown). A dose of 1 mg of ESP7+2CD was ineffective, thus a dose of 3 mgwas chosen to account for the presence of two molecules of cyclodextrin(data not shown).

Equivalents

From the foregoing detailed description of the specific embodiments ofthe invention, it should be apparent that unique chimeric analgesicpeptides have been described. Although particular embodiments have beendisclosed herein in detail, this has been done by way of example forpurposes of illustration only, and is not intended to be limiting withrespect to the scope of the appended claims which follow. In particular,it is contemplated by the inventor that various substitutions,alterations, and modifications may be made to the invention withoutdeparting from the spirit and scope of the invention as defined by theclaims. For instance, the choice of the particular opioid moiety, or theparticular SP moiety is believed to be a matter of routine for a personof ordinary skill in the art with knowledge of the embodiments describedherein.

1-23. (canceled)
 24. A method for treating pain in a mammal, said methodcomprising administering to said mammal a chimeric peptide comprising anagonist opioid receptor binding moiety at its N-terminus and an agonistSubstance P receptor binding moiety at its C-terminus, in an amountsufficient to induce analgesia in said mammal.
 25. The method of claim24 wherein, in the peptide, the agonist opioid receptor binding moietyis a μ, δ or κ agonist opioid receptor binding moiety.
 26. The method ofclaim 25 wherein, in the peptide, the agonist opioid receptor bindingmoiety is a p agonist opioid receptor binding moiety.
 27. The method ofclaim 26 wherein, in the peptide, the N-terminal amino acid residue ofsaid opioid receptor binding moiety is a free amine.
 28. The method ofclaim 27 wherein, in the peptide, the N-terminal amino acid residue ofsaid opioid receptor binding moiety is Tyr.
 29. The method of claim 28wherein, in the peptide, said opioid receptor binding moiety is apeptide having any one of SEQ ID Nos: 1-11, or N-terminal fragmentthereof.
 30. The method of claim 28 wherein, in the peptide, said opioidreceptor binding moiety is endomorphin 1, endomorphin 2, or N-terminalfragment thereof.
 31. The method of claim 30 wherein, in the peptide,said opioid receptor binding moiety is a peptide having SEQ ID No: 2 or3, or N-terminal fragment thereof.
 32. The method of claim 26 wherein,in the peptide, said agonist Substance P receptor binding moietycomprises Substance P, or C-terminal Substance P fragment thereof. 33.The method of claim 26 wherein, in the peptide, the —COOH moiety of theC-terminal amino acid residue of said Substance P receptor bindingmoiety is protected.
 34. The method of claim 33 wherein, in the peptide,the —COOH moiety of the C-terminal amino acid residue of said SubstanceP receptor binding moiety is amidated.
 35. The method of claim 34wherein, in the peptide, the C-terminal amino acid residue of saidSubstance P receptor binding moiety is Met-NH₂.
 36. The method of claim35 wherein, in the peptide, said Substance P receptor binding moiety isa peptide having any one of SEQ ID Nos: 21, 36 and 38-41, or C-terminalfragment thereof.
 37. The method of claim 26 wherein, in the peptide,the opioid receptor binding moiety is endomorphin 1, endomorphin 2, orN-terminal fragment thereof; and the Substance P receptor binding moietyis Substance P, or C-terminal fragment thereof.
 38. The method of claim26 wherein the peptide has SEQ ID No:
 42. 39. The method of claim 26wherein the peptide has SEQ ID No:
 43. 40. The method of claim 24wherein the method of administration is selected from the groupconsisting of intrathecal, intracerebroventricular and systemicadministration.
 41. The method of claim 24 wherein the peptide isadministered with a solubilizing agent.
 42. The method of claim 41wherein the solubilizing agent is cyclodextran.